@Huxley,
My answer is from the point of view of "reality" as a social construction.
All we have are concepts, i.e. nodes of inter-relationship within a semantic network conveyed by a common language. Some of those relationships are enmeshed with the concept of "materiality" and some are not. The concept "theist" and the concept "God" are enmeshed in a particular network in which a positive linkage to "material evidence" may play a role, whereas the concepts "atheist" and "God" have a negative relationship with respect to such a concept of "evidence".
So from this point of view , "existence" is about social agreement regarding relationship between concepts. There are no "things in themselves". All concepts are related. "
Properties of material things" translates as "agreed
expectancies of concurrent inter-relationships between "observer" and "observed". e.g. "tree-ness" implies concurrent "green and brown-ness", "hardness", "shadiness", etc with respect to common human physiology. The implication is that "trees" do not exist as such for non-humans, nor would "trees" exist without the concept of human observers to "thing" them.
Commonality of physiology and social functioning is abstracted within a common language. Language is a priori with respect "higher thinking". It is the
abstract persistence of words which the thinker confuses with the persistence of "things", when in essence all is in flux (particularly " trees" !).
Asking whether X "exists" is futile, the fact that X has already been thinged (conceptualized) has already given it "existence" (relational utility) for at least one "thinger". What we do is negotiate agreement or disagreement about the
utility of X. Beyond that, all we can do when X is cited is say "I don't know what you are talking about". Thus atheists who are prepared to discuss "God"
at all, have already acknowledged existence of the concept (and remember concepts are all we have.)